Hybrid electric vehicles include both an engine and at least one electric motor to provide drive torque to the wheels. There are various types of hybrid electric vehicles. For example, “parallel” hybrid vehicles typically include clutches that selectively enable either or both the engine and the motor to provide drive torque. A “series” hybrid vehicle typically includes an electric motor that is always driveably connected to the road wheels, and an engine that is not mechanically connected to the wheels. In other words, the engine does not provide any of the torque required to propel the vehicle. Rather, in a “series” hybrid vehicle, the engine powers a generator to produce electrical energy that is stored in the battery and/or used by the motor.
The engine of hybrid vehicles can operate to provide drive torque while also charging a battery. To do so, an electric machine, such as an electric motor/generator, can convert output torque of the engine into electric energy to be stored in the battery for later propulsion of the electric machine. If the engine is operating to both propel the vehicle and charge the battery simultaneously, it must have available torque sufficient to do both. In certain situations, the vehicle may have to downshift to allow the engine to produce sufficient drive torque while also charging the battery at a sufficient rate. This can lead to shift busyness with frequent downshifting and upshifting.